GAP 10 PMAGs

+1 on MidwestPX for 10-round 7.62 PMAGs.

Magpul is in the process of manufacturing 10-round 7.62 PMAGs, but they are NOT out on the market yet (hopefully in the coming month or so at most...est. retail will be ~$18/mag).

If you can't find PMAGs right now and can't wait, there's always the metal DPMS LR-308 10-round mags.

Good luck!
 
Unfortunately, we are out of .308 mags to modify. We received a surprise shipment of nearly 1K MAG243s and did one last production run since the SKU was discontinued by Magpul and replaced with MAG291. We won't be cutting down the 291s due to the floorplate design either. Your best bet is to wait for the 10LR (MAG290) which is the factory 10rd mag for large frame ARs.
 
Ustacticalsupply.com is accepting backorders. I placed mine about a month ago and awaiting shipment. They also won't charge you until they ship. Order of 4 came right under $80 with s&h
 
Are the 20 round LR PMAGS too long to shoot prone in a GAP-10? I may need to find some 10 rounders as well...

"It depends" is the short answer (including what kind of ground/terrain you are shooting from, your particular body habitus and what height bipod you are running, etc.).

I personally find the longer, 20-round mags get in the way sometimes but not all the time, but particularly when it comes to mag changes. I personally like shorter mags for prone with all my large-frame ARs from DPMS to POF to Armalite, etc.
 
Can a twenty rounder be cut down to say 15 and the bottom be put back on?

The earlier generation 20LR PMAGs can be so modified (cut-down body/spring, mag baseplate pinned back in place) for a variety of capacities including 15-round. There's a "How To" thread around here somewhere that a quick search should reveal.

The new Gen M3 mags (which replace the now discontinued 20LR) cannot be modified in that fashion because of the redesign that Magpul did on the baseplate...one of many reasons Magpul is now going to sell the reduced capacity 10-rounders.